U.S. Pat. No. 4,622,124 describes an improved horizontal electroblotter used to transfer electrophoretically resolved DNA, RNA or protein from a gel to a receiver such as a membrane. The membrane is a more suitable recording medium to preserve the results of the separation. In this electroblotting process, the gel and membrane are placed in contact in a buffer, between a pair of electrodes, and a direct current is imposed by charging the electrodes with +V and -V. The sign of the charge for each electrode is selected so that the sign on the electrode adjacent to the gel is the same as the net sign of the DNA, RNA or protein to be transferred. In this fashion, the charged DNA, RNA or protein is forced to transfer to the membrane. No attempt is made to reverse the charge on the electrodes.
This process works quite well, as long as the molecular weight of the DNA, RNA or protein is not too large. However, for molecular weights larger than about 2.times.10.sup.4 base pairs, the direct field approach is not adequate to dislodge the high molecular weight material from the gel, at least not in reasonable lengths of time (less than one day).
In addition to transfer via electroblotting, a related transfer process is electroelution, in which there is no need to maintain the DNA, RNA or protein in a specified record of separation. Instead, the DNA, for example, has already been identified, and is needed to be extracted so that it can be purified or cloned. In such cases it is transferred from the gel to a receiver such as a dialysis cup membrane or a salt trap, for example as described in U.S. Pat. No. 4,576,702. In such a case, both the receiver and the gel are placed in a buffer bath so that they are between two electrodes, and a direct current is imposed by charging the two electrodes substantially as described above for electroblotting. Again, such electroelution processes work efficiently only if the molecular weights to be transferred are less than about 2.times.10.sup.4 base pairs.
Chemical elutions have also been tried, in which the gel is melted and the DNA, RNA or protein is extracted by an aqueous phase-oil phase separation. This has proven unsatisfactory, however, as mixing is an essential step in that process, and the mixing leads to shear stress that shears the DNA, RNA or protein unacceptably.
Therefore, there has been a need prior to this invention to provide an electroblotting and/or electroelution process, and apparatus therefore, that can effectively transfer the larger molecular weight DNA, RNA or protein to a suitable receiver.
It has been found that such larger molecular weight materials can be moved within gel for purpose of electrophoretic separation, by altering the fields imposed upon the pair of electrodes, so that the field changes direction. Examples include the processes described in U.S. Pat. No. 4,740,283 and U.S. Pat. No. 4,737,251. However, neither of these patents suggests the use of field changes as a means of bringing about transfer out of the gel. More specifically, the '283 patent, when ready to transfer by electroblot, provides for a direct current flow in only one direction, FIG. 5, just as has always been done in electroblotting, even though the rest of the patent deals with fields that alternate in direction for moving the large molecules through the gel.